1. Field of the Invention
The present invention relates to a rake receiver, and more particularly, to an apparatus and a method for detecting traffic signal transmission in a rake receiver in a CDMA 2000 based mobile communication system in order to detect signal transmission in a traffic channel supporting a discontinuous transmission (DTX) mode.
2. Description of the Related Art
CDMA provides a rake reception function as one of several major characteristics. A rake receiver refers to a receiver capable of separating two signals having a time difference (delay). This function of the receiver can be obtained based upon spread spectrum principle of CDMA.
Multipath fading is a factor having a significant influence on transmission quality in electromagnetic communication. Multipath fading is produced by phase difference (time delay) in signals which have arrived at a receiver through different paths or channels. Such fading decreases the size of a signal which deteriorates Carrier-to-Interference (C/I) ratio, thereby concentrically producing transmission errors.
In addition, time delay creates Inter-Symbol Interference (ISI). Most electromagnetic communication techniques generally overcome fading by using diversity while reducing ISI due to time delay by using an equalizer that compensates for time delay.
Reasons for using a rake receiver in CDMA are as follows. First, it is assumed that three signals have arrived at a receiver at different times through three paths, as shown in FIG. 1. It will be also assumed that path 2 signal has a delay of about 0.5 chip, and path 3 signal has a delay of about 2 chips with respect to channel 1. A chip is an interval of one pulse in spreading code, and discriminated from data bit. One chip is 0.814 μS since CDMA has a spreading rate of 1.2288.
If the rake receiver performs despreading in synchronization with path 1 signal, path 2 and 3 signals are not despread since they are asynchronous with path 1 signal. It is because asynchronous signals are the same when multiplied with different signals or different codes as described above. That is, even though the same three signals arrived with different time delays, only path 1 signal can be decoded through despreading, while other signals which arrived through different paths are not despread. That is, only path 1 signal is extracted from the three signals. Accordingly, if delay of 2 chips is presumed to be synchronous with path 3 instead of path 1, only path 3 signal can be extracted at this time.
That is, CDMA can obtain an effect of time diversity by recognizing signals received with time difference as independent signals. A receiver capable of separating signals which arrive through different paths (or having time differences) as described above is called a rake receiver.
However, as shown in FIG. 1, if a time delay difference between two signals is shorter than 1 chip as in path 1 and path 2, the rake receiver cannot execute signal separation. This, as a result, causes harmful effects to signals. In a mobile communication environment (e.g., a macrocell having a cell radius of about 5 to 20 km), outdoor delay characteristics are reportedly about 2 μs to 3 μs. So, the rake receiver can provide large effects in this environment. However, large effects cannot be produced in a microcell environment or an indoor environment where a time delay is about 0.2 μs. In order to obtain sufficient effect by the rake receiver also in the indoor environment, spreading bandwidth of at least 50 MHz is required.
Description will now be made of a typical rake receiver and its operations with reference to FIG. 2.
FIG. 2 is a block diagram illustrating the concept of a typical rake receiver.
As shown in FIG. 2, a rake receiver includes a plurality of fingers 210, 220 and 230, a plurality of buffers 240, 250 and 260 and a combiner 270. Although three fingers 3 are illustrated, the number of fingers can be selectively increased.
Signals received by a receiving point 200 are input to fingers 210, 220 and 230, respectively, despread and channel-compensated by the fingers 210, 220 and 230, and then as symbols, sent to the combiner.
Each of the fingers 210, 220 and 230 includes a code generator 211 and a despreader 212 as shown in FIG. 2.
When the despreader 212 produces a symbol by despreading an input signal from the receiving point 200 in response to the input of despreading codes generated by the code generator 211, the finger 210 stores the symbol into the buffer 240. The same procedures are performed in the fingers 220 and 230 and the buffers 250 and 260 also.
Since symbols from the fingers 210, 220 and 230 have time differences, they are temporarily stored in buffers 240, 250 and 260 corresponding to the fingers 210, 220 and 230, respectively.
The symbol combiner 270 serves to combine the symbols stored in the buffers 240, 250 and 260 by synchronizing their times.
In the meantime, CDMA 2000 supports discontinuous transmission in a specific traffic channel such as a Dedicated Control Channel (DCCH) and a Supplemental Channel (SCH). That is, it is possible to execute continuous transmission (CTX) or discontinuous transmission in traffic channels according to whether or not data to be transmitted exist in the traffic channels while executing continuous transmission in Pilot Channels (PCH).
Since a transmitting section decides on continuous/discontinuous transmission, a receiving section has to estimate continuous/discontinuous transmission based upon a signal-to-noise ratio in a received signal.
Besides, in order to obtain multi-path diversity gains, CDMA 2000 performs power control based upon a signal-to-noise ratio of a combined signal of signals received via respective finger paths in a rake receiver. So, CDMA 2000 estimates continuous/discontinuous transmission of a traffic channel based upon signal-to-noise ratio obtained by combining the signals received according to the fingers.
However, in detecting continuous/discontinuous transmission of the traffic channel by using the signal-to-noise ratio as above, a signal-to-noise ratio in continuous transmission does not have sufficiently spaced distribution compared to that in discontinuous transmission. Therefore, as one of the drawbacks, there is high error rate.